An Introduction to the Serpentine Plant Community of the Putah-Cache
Bioregion

Kelly G. Lyons

I once pictured myself working in a mild mountain field site,
recording data under a nodding Douglas fir, with my hair lightly windblown.
It is perhaps because of my naiveté as a blossoming field ecologist that I haven't
always appreciated serpentine habitats. Serpentine vegetation is dwarfish so
if you want to see it you often have to squat. There is very little shade and
the rocky, exposed soil can get so hot you can feel it through the soles of
your shoes. Sometimes when I hike these landscapes they feel so remote that
I imagine they are best suited for disposing of a corpse.

It was not until I found a good guide who showed me how to
see serpentine in a more enlightened way that I paused long enough to learn
its secrets. On serpentine, you can find unspoiled habitat, beautiful "mini"
flora, bubbling springs, swimming holes, bobcat tracks and solitude. I now look
forward to exploring these landscapes every season. Here I offer a brief introduction
to serpentine in the Putah-Cache bioregion but, ultimately, nothing can replace
first hand exploration to tease the subtle sublimity from serpentine's harshness.

Recognizing Serpentine

Serpentine is rare on the surface of the earth but common in
California and the Putah-Cache bioregion. About 13% of the geologic substrates
in the bioregion are serpentine. Frequent seismic activity and proximity to
the ocean account, in part, for their abundance. Serpentine habitat is easy
to recognize because of its distinctive green-gray, shiny rocks, which are often
mottled black and streaked white, and sparse, tough vegetation. Over time, weathering
oxidizes the iron in the rocks, rendering the soil burnt-orange.

During their time below the earth's surface, serpentine rocks
are under high pressure. Once exposed, the rocks break down into layered, unstable
talus. If you examine serpentine talus you find that each edge is very different.
Where it is smooth and dense it has been placed under great pressure, and you
can feel the direction in which the rocks slid past one another by rubbing your
fingers along the smooth surfaces; they feel scaly.

The Serpentine Syndrome

Serpentine outcrops have been referred to as barrens because
they are often sparsely vegetated and serve few of our agricultural needs. Inhabitants
of these habitats experience drought, nutrient stress, heavy metal exposure
and exposure to high light. Serpentine is severe because of the underlying substrate.
Soil derived from serpentinitic rock presents so many hardships to plant life
that Hans Jenny coined the term serpentine syndrome to refer to them collectively.
The "syndrome" influences the overall biomass (amount of living material) on
serpentine, which is low compared to other plant communities. Nonetheless, many
species, equipped with unique physiologies to withstand the stress, thrive on
these soils.

Serpentine, like most igneous rocks, contains iron and magnesium
mixed with silica, but iron and magnesium are unusually high in these substrates.
Soil scientists call this condition ultramafic ("ma" stands for magnesium and
"f" for ferrum, or iron). Serpentine soils are also high in heavy metals such
as chromium, cobalt and nickel. While serpentine outcrops differ in their concentrations
of these metals, many sites contain levels that are toxic to many plant species.
Serpentine is also rich in silica fibers known as asbestos although these minerals
appear to pose more of a problem for humans than plants.

Another challenge serpentinic soil poses is a lack of nutrients.
Calcium is unusually scarce and, for these soils, is reported in terms of its
ratio to magnesium. Many scientists believe this ratio is a key factor in determining
plant survivorship. Both elements are positive ions that compete for the same
uptake sites in plant roots. When the concentration of magnesium is high, these
uptake sites become saturated, making it harder for plants to obtain calcium.
Low levels of calcium exacerbate this condition. Other important nutrients lacking
in serpentine include potassium and phosphorous, which are both rapidly lost
through weathering and leaching. Nitrogen is also scarce due to limited plant
growth.

Finally, serpentine soils are thin. This means there is less
substrate on which nutrients and water can be held and made available to plants.
Deep serpentine soil occurs only in valleys, in alluvial soil, where rains wash
small particles downward. Vegetation in these valleys is denser, but the soil
is still ultramafic. The assumption that valley soils are richer because they
support more biomass has misled those who tried to use them for grazing or agriculture.
Successful ranching on serpentine is shortlived because removal of the vegetation
results in rapid loss of cover and the eventual loss of water and minerals.
Most farmers don't bother.

Serpentine and Plants

Serpentine species are generally smaller than their non-serpentine
counterparts and, as with most plant species, can be lumped into two categories:
tolerators and avoiders. They either put up with the dry conditions or avoid
it. The tolerators are the dominant vegetation. These are primarily low-growing
shrubs and occasional large trees with tough leaves that look silvery or flat
gray due to hairs designed to reflect light. The avoiders germinate from seed
or sprout from underground storage structures and complete their lifecycle while
water is available. Many avoiders require close examination to be appreciated.
Squatting works best. In closer proximity you can see patterns on the tiny petals,
veins in the leaves, and miniature fruits.

Endemic Species

Endemism occurs when a species is found exclusively in one
designated location and no other. These locations can be defined naturally,
by environmental conditions, or culturally, by lines of a map. For example,
a species could be a serpentine endemic, found only on serpentine, or endemic
only to Lake County. In some cases both occur simultaneously.

Serpentine habitats are geologic islands in a sea of other
soil types. When these rocks were exposed, new species dispersed on them from
the surrounding substrates. Eventually, those that could colonize and survive
on serpentine evolved on a separate trajectory from their non-serpentine relatives.
In many cases, the new species survived on a patch of serpentine because they
were poor competitors on other substrates. As a result of this island effect,
serpentine substrates house a large number of species that are found only on
serpentine and have highly restricted ranges.

Approximately 282 serpentine species are listed by the California
Native Plant Society as rare. Some of these are threatened due to anthropogenic
causes, but many are rare simply because they are unusually restricted in their
distribution. And, while they are not found in many locations, they may be locally
abundant. Unlike rare species that have faced an unnatural contraction in their
numbers, serpentine endemics are rare by nature. This suggests that for many
serpentine species conservation is straightforward. Set the land aside, and
as long as it remains undisturbed the plants will persist as they have for thousands
of years.

Bare monkey flower (Mimulus nudatus) is an example
of a highly restricted, serpentine endemic. This small seep plant with large
pouting yellow flowers is found only in Lake and Napa counties. In rainy years,
when serpentine seeps are well saturated and the growing season is prolonged,
its populations are large. It may be confused with common yellow monkey flower
(M. guttatus), which also grows on serpentine. But bare monkey flower
has fewer leaves and that are shaped like thin feathers rather than spades.

Large and widespread serpentine endemics can act as flag species.
Because they are loyal to the substrate, you know you are on serpentine when
you see them. The most abundant and conveniently large flag species in our bioregion
is leather oak (Quercus durata var. durata). This is a stumpy,
dense shrub with small, dark green, cupped leaves that may be spiny on the margins.
The acorns are oval with rounded tips. The common name is easy to remember once
you have come in contact with a specimen because the leaves are tough and leathery.

Other common endemic shrubs include musk brush (Ceanothus
jepsonii var. albiflorus) and white leaf manzanita (Arctostaphylos
vicida ssp. pulchella). Musk brush is a California lilac
that is restricted to the Inner Coast Range. It has gray stems with spiny leaves
and tight clusters of tiny white flowers. Manzanita species have reddish bark,
elliptic or oval leaves and small red berries that, as the Spanish derived common
name implies, look like little apples. White leaf manzanita is an erect shrub
with generally gray leaves and sticky stems and flowers.

The cypresses that grow on serpentine in the Putah-Cache Creek
Bioregion are also distinctive and easily recognized. McNab cypress (Cupressus
macnabiana) is a widespread species that is relatively loyal to serpentine
soils in our watershed. You can tell it is a cypress because it has minute,
awl-shaped leaves and round woody cones with shield-shaped scales, similar to
an interior redwood. Sargent cypress (Cupressus sargentii) is another
woody species commonly found on serpentine in the bioregion. It can be distinguished
from McNab cypress because the resin glands (small dots) on the tiny scale leaves
are located at the base, instead of the middle, of the leaf.

Some genera are particularly well represented on serpentine.
For example, half of the 24 Californian species of the genus Streptanthus
are associated with serpentine soils. Although easily overlooked due to their
spindly growth habit and cryptic (camouflaging) colors, these herbaceous plants
are distinctive. In early spring, the plant displays a basal rosette, a whorl
of leaves growing near the ground. When ready, the apex expands producing a
series of small flowers along a tall stem. The wavy petals protrude, sometimes
ever so slightly, from an urn-shaped whorl of chartreuse and/or purple sepals.

Art Shapiro of UC Davis discovered a rare form of mimicry in
the serpentine species Streptanthus breweri. These plants form small
protrusions on their leaves that resemble the eggs of pierid butterflies. This
is an effective defense mechanism because it deters butterflies from laying
their eggs where they believe their herbivorous larval offspring would have
to compete with other butterfly larvae for resources.

Bodenvag Species

Not all species growing on serpentine are restricted to these
soils. Taxa with the ability to grow on and off serpentine are called bodenvags,
a German word meaning indifferent. This indifference allows them to be more
common and widely distributed. In the Putah-Cache bioregion our most prominent
bodenvag is Pinus sabiniana, also known as gray, foothill and, my personal
favorite, ghost pine. This is a tall, branching pine with gray needles. The
tree looks light and airy and appears to hover in the landscape, especially
at dusk.

Two common chaparral shrubs of the rose family, chamise (Adenostema
fasciculatum) and toyon (Heteromeles arbutifolia) are easily recognized
bodenvags. In late spring, chamise's small, white flowers blanket the bush.
This pervasive chaparral species is often found in dense stands on south-facing
slopes. Its short needle-like leaves aid in water conservation allowing it to
tolerate dry conditions. Toyon's white flowers emerge later in the season. It
has deep green, elliptical (eye-shaped) leaves with serrated margins. It is
also called Christmas berry because it dons bright red fruits in fall and winter.

Other notable shrubs are flannel bush (Fremontodendron
californica ssp. napense) and silk tassel (Garrya congdonii).
Flannel bush, a member of the cacao or chocolate family, has salmon yellow flowers
that stand out in dramatic contrast to the dark vegetation. "Flannel" describes
the leaves, which have star-shaped hairs that can irritate your skin if you
handle the leaves. Silk tassel might be mistaken for an oak, but if you are
lucky enough to catch it in bloom, you will find that the female flowers resemble
a drooping stack of bells and the fruits balls on a string.

Herbaceous bodenvags easily recognized from the speeding car
include the herb Globe gilia (Gilia capitata) and the grass big squirreltail
(Elymus multisetus). Globe gilia has round clusters of small dusty-purple
flowers at the tips of tall stems. The inflorescence (cluster of flowers) of
big squirreltail is bushy with an airy feel. If you try to grab the flowering
stalk of a dried late-season specimen the fruits will separate, becoming an
amorphous clump.

A Guide to Serpentine in Our Bioregion

The best way to learn about serpentine is to explore it on
your own. You will find the highest density of flowers from late February through
June, although all months offer something interesting. In August and September
the late-season sunflowers and tarweeds are beautiful, but it can be unbearably
hot.

The following list provides areas for hiking, swimming, mountain
biking, hunting, ORV use and botanizing. Dogs are prohibited on the UC Reserve
but they are allowed on BLM and California Department of Fish and Game (CDFG)
property. Pay close attention to the signs and avoid trespassing on Homestake
property. Also, watch for researcher's flagging so that you, your children or
your dog doesn't trample someone's research. Hunters actively use the public
lands so you might want to take precautions and wear some orange. Finally, be
sure to bring plenty of drinking water.

Guenoc Winery and Vineyard: They have a wine-tasting room,
a picnic area hedged by leather oak, and a nice view of the surrounding lands.

Indian Valley Reservoir (BLM): From Hwy 20 go north on Bear
Valley Rd. (Although not serpentine, Bear Valley is a great place to view wildflowers
in the spring.) Follow Bear Valley Rd. past the Wilbur Hot Springs turn-off,
to Brim or Bartlett Springs Rd. and turn left. This undeveloped road will take
you onto a large serpentine intrusion. If you continue west you will reach the
north end of Indian Valley Reservoir. I like this area because it is so remote
and the reservoir water is pleasantly warm for swimming. You may access the
south end of Indian Valley Reservoir from Hwy 20, west of the Bear Valley Rd.
turn-off.

Knoxville Public Lands, McLaughlin Mine and UC Natural Reserve
(BLM, Homestake and UC System): Follow Knoxville-Berryessa Rd. along the lake
and north along the Eticuera and Knoxville creeks. This is a somewhat rough
road with low water crossings. The road takes you through several stands of
calmingly beautiful blue-oak woodland. At 19 miles past the Knoxville-Berryessa/Pope
Canyon junction, you come to the Knoxville-Devilhead Rd. Take a left onto Knoxville
Public Lands. The road is bumpy and, when not well maintained (usually in winter),
best navigated with a high-clearance vehicle. It leads to a parking lot where
you can leave your car and hike.

The Cedar Roughs (BLM and CDFG): From the Knoxville-Berryessa/Pope
Canyon junction turn left (west) on Pope Canyon Rd. Follow it for 2.2 miles
and park on the south side of the road near a brown and yellow gate. Follow
Pope Creek upstream for approximately 1/4 mile. Look for trail markers. At the
marker, cross the creek and climb the steep hill to an abandoned jeep trail.
This road will take you onto the Cedar Roughs. The Cedar Roughs are remote and
rugged. Watch for ticks and rattlesnakes.

References

If you wish to learn more about serpentine flora and do more
serious botanizing on your own the two publications below are very useful. Both
may be purchased through the California Native Plant Society (www.cnps.org).